In the past, high-purity silicon material was primarily used for the production of semiconductor components. With the development of semiconductor integrated circuit technology, the integrated circuits have become smaller. Thus, the consumption of high-purity silicon has not been significantly increased even though there are more electronic devices on the market.
The demand for high purity silicon has grown significantly in solar photovoltaic industry in recent years as the high purity silicon is also an important raw material of solar photovoltaic cells. The demand in the solar photovoltaic industry has now exceeded the demand in the semiconductor industry. However, the profit margins for solar photovoltaic industry are very small. It is imperative to reduce the cost of high-purity silicon material production. The high cost has made the traditional methods of production a great challenge.
Traditional methods of producing high purity silicon include the Siemens method and the fluidized bed method. The Siemens method includes a process of introducing the purified silicon containing gases such as trichlorosilane (SiHCl3) or silane (SiH4) and hydrogen gas mixture into a reactor, inducing a thermal decomposition reaction on the surface of hot mandrel and continuously thickening the high purity silicon by deposition of silicon on hot mandrel surface, and returning the exhaust gas into the treatment system for separation and recycling. When the diameter of the silicon mandrel grows to a certain point, the reaction has to be terminated and the silicon mandrel is replaced for the next round of reaction. The process involves intermittent operation and requires high energy consumption. One kilogram of high-purity silicon consumes 150 kwh on average or even higher. In addition, there are other disadvantages such as low efficiency. The production of high purity silicon by the Siemens method leads to low yield and high cost and it will not meet the growing needs from the solar industry.
The fluidized bed process includes heating high purity granular silicon seeds in a reactor, introducing high-purity silicon containing gas into the reactor; inducing thermal decomposition on the surface of the seeds so that high purity granular silicon grows bigger and bigger and precipitates in a collection box. As the fluidized bed process uses a large quantity of high purity granular silicon seed, the entire surface area is relatively large compared to Siemens method. The reaction efficiency and conversion efficiency are greater than for the Siemens process; and the power consumption is also reduced. Nevertheless, certain issues remain with the conventional fluidized bed method including the following.
1. High purity silicon granules formed in the suspended state separate from each other forming more than 80% free-space. Consequently, a large amount of silicon fine powders are formed from the decomposition of silicon-containing gas and are taken away with the reactor exhaust gas therefore reducing the raw material (gas) usage and increasing costs. Additionally, the silicon fine powders increase the difficulty of the treatment of the exhaust gas as well as the cost of equipment as the silicon powder enters the downstream process. The silicon fine powders also cause pollution.
2. In a fluidized bed reactor, suspending silicon particles, especially large sized particles, consume a large amount of gas in the reactor that results in the difficulty of gas recovery. In addition, it leads to low utilization of reaction heat and increased operating costs.
3. The surface of silicon granules is in a semi-molten state under the reaction temperature (200° C.-1400° C.). Due to the adhesion between particles, the semi molten state causes inter-particle agglomeration. Consequently, the reactor distributor pores, pipings and channels are easily plugged and result in cut-off accidents.
4. The cost of equipment is high and construction of equipment is difficult due to large reactor volume required, the less effective use of space. Only small-scale production is practical in operation.
5. The preparation of high purity granular silicon seed is difficult as it can be easily contaminated during the preparation.